Field of the Invention
The present invention relates to a phase locked loop for recovering a clock signal from a data signal, having a delay locked loop with a phase detector with a first input that is coupled to a connection for supplying a signal that can be derived from the clock signal, and with a second input that is coupled to a connection for supplying the data signal, with an integrator that is connected to one output of the phase detector, and with a delay element that is connected by a control input to one output of the integrator and that is connected on the output side to one of the two inputs of the phase detector, a loop filter that is connected to the output of the integrator, and a voltage controlled oscillator that is connected on the input side to one output of the loop filter and at whose output the clock signal can be tapped off.
Recovery of a clock signal from a received data signal, for example, a binary signal with a random sequence of zeros and ones, is a central problem in data technology and telecommunications technology.
One possible solution approach is to use a phase locked loop with a digital phase detector, which produces an actuating signal for a local oscillator. In such a case, the phase angle of the data signal is compared with the clock phase of the clock signal in a digital phase detector of this type in each case with respect to the flank changes in the data signal, that is to say, the changes from logic 0 to logic 1 and vice-versa The phase detector in such a case produces at its output the information xe2x80x9cclock too earlyxe2x80x9d, xe2x80x9cclock too latexe2x80x9d, or xe2x80x9cclock correct or phase unknownxe2x80x9d. This signal information is used for keying a frequency of an output signal of a local voltage controlled oscillator (VCO), and, thus, for following the phase angle of the data signal. This principle is specified, by way of example, in the article xe2x80x9cClock Recovery from Random Binary Signalsxe2x80x9d, J. D. H. Alexander, Electronics Letters Vol. 11, No. 22 (1975), pages 541 to 542 as well as in the article xe2x80x9cSi Bipolar Phase and Frequency Detector IC for Clock Extraction up to 8 Gb/sxe2x80x9d, A. Pottbxc3xa4kker, U. Langmann, IEEE Journal of Solid-State Circuits, Vol. 27, No. 12 (1992), pages 1747 to 1751.
The use of a digital phase detector in a phase locked loop PLL for obtaining a clock signal from a data signal can be implemented quite easily in terms of the circuitry. The digital or nonlinear method of operation of the phase detector is, however, a disadvantage for the transmission system in comparison to a linear method of operation because, in the event of any phase error, only its mathematical sign is known, but not the magnitude of the discrepancy. In consequence, it is not possible to specify a linear transfer function for the system or a modulation bandwidth for the phase modulation. However, because the transmission of data over long distances is a frequent objective in telecommunications technology, in the process of which a large number of signal regenerators have to be connected in series, it is desirable for the circuits used for clock recovery to be linear and to have a well-defined modulation bandwidth.
German Published, Non-Prosecuted Patent Application DE 198 42 711 A1, corresponding to U.S. Pat. No. 6,433,599 to Friedrich et al., discloses a circuit for data signal recovery and for clock signal regeneration in which, in addition to the PLL for clock recovery with a digital phase detector, a second PLL is provided and has a linear, analog phase detector, is connected downstream from the first PLL, and produces an output clock signal from the clock that is produced in the first stage. However, such a circuit requires a second voltage-controlled oscillator with the additional complexity that is associated therewith.
The article xe2x80x9cA 155-MHz Clock Recovery Delay- and Phase-Locked Loopxe2x80x9d, T. H. Lee, J. F. Bulzacchelli, IEEE Journal of Solid-State Circuits, Vol. SC-27, December 1992, pages 1736 to 1746, discloses a circuit of this generic type in which a delay locked loop DLL is combined with a phase locked loop PLL, connected in parallel. It is, thus, possible to achieve very fast clock signal recovery with high performance and good jitter characteristics. The phase detector that is used may, in such a case, assume two or more output values, for example, five output values, which are integrated in a loop integrator to form a triangular waveform signal.
The loop filter in the control loop that is described in the article has a pure integrator without any proportional component, see FIG. 9, with the function Hf=KD/s. The output of this loop filter is connected to a voltage-controlled oscillator VCO. This VCO must be a high-precision crystal oscillator (VCXO), whose frequency differs only insignificantly from the data rate. Any difference between the oscillator frequency and the data rate of the data signal must be compensated for by a steady-state actuating value for the loop filter, which also controls the controllable delay element. This restricts the phase control range of the delay loop, as is explained in the Chapter xe2x80x9cC. Acquisition Behavior of the D/PLLxe2x80x9d.
The described D/PLL is configured by the two poles of the phase transfer function (jitter transfer function) H(s), see Chapter B, which can be adjusted by the DLL parameters KD and K"PHgr" as well as the PLL parameter K0. However, linear components, in particular, a linear phase detector with a defined detector constant KD, are required for correct configuration of this linear function. This phase detector must be able to make a quantitative statement relating to the phase error, in addition to a qualitative statement.
It is accordingly an object of the invention to provide a phase locked loop for recovering a clock signal from a data signal that overcomes the hereinafore-mentioned disadvantages of the heretofore-known devices of this general type and that makes possible a linear phase locked loop whose design has been simplified further.
A clock signal normally has a predefined known sequence of binary coded series of zeros and ones, which, normally, also alternate.
In contrast, a data signal carries coded information that, for example, is not known a priori to a receiver, such as speech data, text data, graphics data or other data. Thus, even if the use of a scrambler makes it possible to achieve an equal probability of the occurrence of zeros and ones when averaged over a lengthy time period, it is not necessary to know, for example, at the receiver the clock information on which the data signal is based. In consequence, the recovery of a clock signal from a data signal is of major importance in information and communications technology.
With the foregoing and other objects in view, there is provided, in accordance with the invention, a phase locked loop for recovering a clock signal from a data signal, including a delay locked loop having a connection for supplying a signal to be derived from the clock signal, a connection for supplying the data signal, a nonlinear phase detector having a first input coupled to the connection for supplying the signal, a second input coupled to the connection for supplying the data signal, and at least one output, the nonlinear phase detector producing, at the at least one output, one of a signal able to assume one of only three states including a first state in which a phase of the clock signal leads a phase of the data signal, a second state in which the phase of the clock signal lags the phase of the data signal, and a third state in which a phase angle of the clock signal and a phase angle of the data signal one of match and are instantaneously unknown, and a binary signal, an integrator having an output and an input connected to the at least one output of the phase detector, and a delay element having a control input connected to the output of the integrator and an output connected to one of the first and second inputs of the phase detector, a loop filter having an output, an input connected to the output of the integrator, a proportional regulator component, and an integral regulator component, and a voltage controlled oscillator having an input connected to the output of the loop filter and an output at which the clock signal is to be tapped off.
According to the invention, a phase locked loop for recovering a clock signal from a data signal is provided and is developed such that the phase detector is a nonlinear phase detector.
The nonlinearity or digitality of the phase detector is distinguished, in particular, in that, although the phase detector makes a qualitative statement as to whether the phase error between two input signals is positive or negative, it does not make any quantitative statements on the magnitude of this phase error. Such phase detectors are also referred to as xe2x80x9cbangxe2x80x94bang detectors.xe2x80x9d They are distinguished, in particular, in that their circuitry can be configured with a relatively low level of complexity.
The output of the phase detector may, in this case, produce a signal that, for example, can assume three values, namely, clock too early, clock correct, or clock too late, depending on whether the phase angle of the clock signal leads or lags that of the data signal, matches it, or is instantaneously not known. A signal such as this at the output may be a ternary signal, which has a positive value when the phase difference has a positive mathematical sign, a negative value when the phase difference has a negative mathematical sign, or the value zero when the phase difference is zero or cannot be determined at that time. The signal at the output, however, does not provide any quantitative statement about the magnitude of the phase difference.
Alternatively, a binary signal can be produced at the output of the phase detector, providing either a logic zero or a logic one depending on whether the phase difference has a positive or negative mathematical sign.
This combines the advantages of the PLL combined with a DLL, which allows high performance, with the advantages of simple configuration and simple implementation of a digital phase detector. The delay locked loop DLL with the digital phase detector as well as the integrator and the delay element that may be configured to be controllable in this case, overall, represents a circuit element whose electrical characteristics correspond to those of a linear, analog phase detector.
According to the present principle, a nonlinear phase detector is used to compare a data signal that arrives in the circuit with a clock signal. In such a case, either the data signal or the clock signal is supplied to the phase detector with a delay. The phase detector may produce an actuating signal, for example, a ternary actuating voltage at its output, which is used to drive an integrator that is connected downstream from the digital phase detector. To form a DLL, the output of the integrator is connected to a delay element that is located either in the data path or in the clock signal path, on the input side of the digital phase detector. The delay element may, in this case, be in the form of a controlled delay element. In such a case, the delay is controlled by the signal that is produced at the output of the integrator.
This control loop forms a delay locked loop DLL. In this case, either the clock phase is slaved to the data phase or the data phase is slaved to the instantaneous clock phase in a nonlinear, very fast, control process. The output signal from the DLL that is produced at the output of the integrator in this case depends in a linear form on discrepancies between the clock phase and the data signal phase, provided that the delay element that is connected to one input of the digital phase detector has a linear characteristic.
In the phase locked loop, the signal that is produced at the output of the integrator is, now, filtered in a loop filter that is connected downstream from the integrator, and controls a voltage controlled oscillator that is connected downstream from the loop filter. The loop filter may, in such a case, have a proportional component, which can be used to configure the bandwidth of the PLL, and an integral component, with which the remaining control error between the data signal phase and the clock signal phase can be made to be equal to zero, or can be made as small as possible.
In accordance with another feature of the invention, the loop filter that is connected downstream from the integrator has a proportional regulator component. This proportional component is used for the actual phase control process. However, to produce a second order phase transfer function in the proposed configuration, the loop filter has an integral component, which introduces the second pole of the transfer function, rather than using the delay loop. In such a case, the integration constant of the integrator is negligibly small. Because time processes in the delay locked loop are always negligibly short in such a case, the phase detector does not need to have a linear response. It is, thus, possible to use a simpler, nonlinear phase detector.
The two poles of the phase transfer function in the present configuration can be configured using the parameters of the phase locked loop without any defined or linear output value of the phase detector being required for such a purpose.
The phase transfer function of one preferred embodiment of the invention is, in this case:       H    ⁡          (      s      )        =      1          1      +              s        ·                              K            r                                              K              0                        ·            F                              +                        s          2                ·                  T                                    K              0                        ·                          K              d                        ·            F                              
where F is the transfer function of the loop filter, Kxcfx84 is the conversion gradient of the delay element (phase/voltage), K0 is the conversion gradient of the voltage controlled oscillator (circular frequency/voltage), Kd is the phase detector constant (voltage/phase), s is the complex circular frequency, and T is the integration time constant of the integrator.
If the integration time constant T is assumed to be negligibly short, this results in the phase transfer function H(s) becoming       H    ⁡          (      s      )        =      1          1      +              s        ⁢                              K            r                                              K              0                        ⁢                          xe2x80x83                        ⁢            F                              
This does not include the detector constant Kd, in contrast to the phase transfer function Kclassical(s) of a conventional PLL:             H      classical        ⁡          (      s      )        =      1          1      +              s        ·                              K            r                                              K              0                        ·                          K              d                        ·            F                              
As in classical PLL theory, H(s) in the present configuration is second order if the transfer function F is a first-order piecewise rational function, that is to say, it has an integral component. With the proposed configuration KD, which is undefined for a nonlinear or bangxe2x80x94bang phase detector, can be replaced by the expression 1/Kxcfx84 for configuration purposes, with the second-order control loop being configured as a linear system even though the phase detector is nonlinear.
A further advantage of a loop filter with an integral component is that any discrepancy between the VCO frequency and the data rate of the data signal can be compensated for by this integral component. After completion of such a control process, the delay locked loop can be operated with the same drive range as that which is possible without any frequency error. There is, therefore, no need for a high-precision crystal oscillator. In fact, it is even possible to use a voltage-controlled oscillator VCO that can be tuned over a wide range. This is important, especially for high data rates in the Gigahertz range, because it is impossible to produce crystal oscillators for such high frequencies.
In accordance with a further feature of the invention, the delay element is connected between the connection for supplying the data signal and the second input of the phase detector. The configuration of the delay element in the data path is one possible implementation of the proposed principle, which allows a particularly simple circuit configuration.
In accordance with an added feature of the invention, in which the delay element is disposed in the data path, one data input of the delay element is connected to the output of the integrator, in order to control it.
In accordance with an additional feature of the invention, the delay element is connected between the output of the voltage controlled oscillator and the input of the phase detector. The delay element is, in such a case, disposed in the clock path of the circuit.
In accordance with yet another feature of the invention, if the delay element is disposed in the clock path, it is connected to the output of the integrator, in order to control it.
In accordance with yet a further feature of the invention, if the delay element is disposed in the clock path, a further delay element is connected to the output in order to provide a clock output signal. In such a case, it is advantageous for the delay element of the further delay element to be shorter than a lower limit of an adjustment limit of the delay time of the delay element in the clock path.
In accordance with yet an added feature of the invention, if the delay element is disposed in the clock path, a matching cascade circuit having at least one matching delay element is provided for the phase detector and integrator, for matching the data signal phase to the phase angle of the signal that can be tapped off at the oscillator. In consequence, the tolerance range with regard to jitter can be extended as far as the limits that are set by the fast delay locked loop.
In accordance with a concomitant feature of the invention, the integrator is in the form of a low-pass filter.
Other features that are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a phase locked loop for recovering a clock signal from a data signal, it is, nevertheless, not intended to be limited to the details shown because various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof, will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.